you mean "slow motion" ? i.e., original video is 30 frames/second, but playing with 20 frames/s, or 10 frames/s. so looks like slower than normal. if it's the case, I haven't experienced it yet.
do the video play normally without seek? it's normal in my random seek tests.
to prevent a single IP consuming too many bandwidth, maybe you can try limit the Outbound Bandwidth to 256K(or 364K, 128K etc), to see if any improvement.
Server->Security->Outbound Bandwidth (bytes/sec)